A varistor is a passive electronic component which has the characteristic of nonlinearity with respect to I-V (current-voltage). Varistors are used mainly for overvoltage protection and voltage stabilization. ZnO quickly became the leading varistor material because of its excellent nonlinear characteristics, after being successfully developed in 1968 by Panasonic.
The chip varistor emerged in 1981 which first reported by Panasonic. This chip varistor used technology relating to laminated ceramic green sheets and platinum (Pt) inner electrodes; since that time, ZnO-based varistor featuring low-voltage multilayer chips have been successfully developed. TDK, Mitsubishi, EPCOS and some other companies have undertaken sustained research on the multilayer chip varistor. At the beginning of this century, the 0402-package multilayer varistor was developed in succession by companies including AVX, TDK, LITTELFUSE, AMOTECH, EPCOS and some others. Murata and Panasonic have developed the chip varistor with a smaller 0201-package geometry; the breakdown voltage of this chip varistor is as low as 2.5V, and it can meet the ESD protection requirements of semiconductor devices of different performances and structures. The chip varistor has been researched extensivelr in recent years, with remarkable results being achieved in the basic research on chip varistor materials, as well as its precision manufacturing processes.
Currently single layer varistors typically have a thickness of 1 mm or so, with the film thickness of each layer of ZnO multilayer chip varistor typically being as thin as several tens of micrometers,which allow the breakdown voltage to be reduced by reducing the amounts of grains in single film. National Taiwan University reported the multilayer chip varistor wherein each layer has only 1-2 grains and wherein the thickness is 8 μm after being sintered. Ceramic green sheets having such thickness can also increase the average grain size of ZnO by increasing the sintering temperature, lengthening the sintering time, adding sintering aids, and so on, in order to reduce the breakdown voltage. Currently, multilayer chip varistors mainly employ systems of ZnO materials, with precious metals, such as silver (Ag), palladium (Pd) etc., as inner electrodes, and using a preparation method invoving firing in air.
With the decrease of layer thickness and the increase numbering number of layers, the mass proportion of the materials comprising the inner electrode within low voltage multilayer chip varistor is growing. Because the sintering temperature of ZnO varistor materials is generally higher than 1000° C., the high melting point alloy Ag/Pd (with a molar ratio of 30:70)must by used as the inner electrode material; this alloy accounts for over 50% of the total varistor cost. Further, the sintering process involves a ZnO—Bi2O3 system, with Bi2O3 being highly volatile and prone to reacting with the Pd electrode material, thereby reducing device performance. Many researchers and manufacturers are studying the replacemeny of the Ag/Pd inner electrode with cheaper materials, such as silver (Au) and cooper(Cu), by reducing the sintering temperature of the ZnO chip varistor. In this century, major companies have been attempting to reduce varistor costs as a result of the high costs associated with multilayer chips varistor prepared by this method, in particular due to the sharp increase of Pd and some other noble metal electrodes. Lavrov demonstrated that Cu can be co-fired and can be compatible when used as an electrode with a ZnO varistor ceramic, but the preparation method is very complex (Lavrov et al., Journal of the European Cermaic Society, 24 :2591-5 (2004)); in 2011, Changzhou Star John Technology Co., Ltd. announced a patent about the preparation of ZnO varistors wherein the Ag/Pd electrode is replaced by an electrode of pure Ag, but the cost of Ag electrodes remains high. Methods of reduction and re-oxidation have been used to prepare a multilayer chip varistor using as system of SrTiO3 materials, wherein the base metal is used as the inner electrodes, as in the patent JP2002222703A of TDK, patent JP2005085780A of Panasonic and the patent US20070273468. However, these and other methods suffer from the very low varistor nonlinear coefficient (below 10), which limits the field of potential varistor applications.